Manufacture of oxide-coated cathodes



Patented Nov. 15, 1927.

sures 1,648,941 PATENT OFFICE.

WILLIAM BENJAMIN GERO AND GEORGE WILSON HALLOCK, OF BLOOMFIELD, NEW JERSEY, ASSIGNORS, TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

MANUFACTURE OE OXIDE-COATED CATHODES;

No Drawing.

This invention relates to cathodes for electron discharge devices employing as the active electron emissive material a coating of the oxides of the alkaline earth metals as, for instance, a mixture of barium and strontium oxide.

In the manufacture of cathodes of the above nature it is the customary practice to coat a metallic base such as a platinum iridium filament with a compound of the alkaline earth metals other than the oxide and to convert the same to the oxide either before or after sealing ofthe cathode into an envelope of the device.

In an application of Duncan MacR-ae, Se-

rial No. 587,270, filed September 11, 1922, entitled Electron-emitting devices and method of making and assigned to the VVestinghouse Lamp Company there is disclosed a' method of coating a metallic base material with the carbonates of the alkaline earth metals, such compound being baked onto the filament in the presence of carbon dioxide in order to prevent decomposition thereof or conversion of the carbonates to the oxide.

After the cathode is enclosed in the evacuated envelope it is heated by passage of current therethrough to convert the carbonates to the oxide and thereby render the cathode thermionically active. It has been found that decidedly superior life and emission are obtained when the carbonate coating of the filament is not decomposed prior to its inclusion in the evacuated envelope. The carbonates moreover are extremely stable in air and the filaments coated therewith may be stocked for long periods of time without deleterious contamination or alteration of the emissive properties thereof.

According to the process set forth in the application of MacRae supra, a coating of the carbonates of the alkaline earth metals is obtained by applying the same from an aqueous suspension of the alkaline earth carbonates, each coating so applied being baked onto the filament. Any desired number of coatings may beapplied depending upon the consistency of the suspension and the Weight of coating required on the filament.

The present invention is concerned with the production of a cathode in accordance with the process of the MacRae application, an object of the invention being to cause a. firm adherence of the carbonates to the me- Application filed July is, 1926. Serial No. 123,031.

tallic core so that they will resist abrasion and handling during factory manipulations such as mounting.

Other objects and advantages will hereinafter appear.

We have found that when a soluble salt such as potassium permanganate is added to the aqueous suspension in the proper proportion that it greatly facilitates the coating operation making the resulting coating adhere very much better to the metallic base. A coating composition which we have found to be particularly efficacious in the production of oxide coated cathodes is as follows:

Barium carbonate, 8- grams; strontium carbonate, 8 grams; potassium permanganate, 30 cc. of a 2% aqueous solution.

The materials are thoroughly mixed and applied to the filament in any suitable manner, such as by spraying,'dipping, or painting but preferably the filament is drawn through the suspension in a continuous manner. This may be conveniently carried out by the use of a gettering cup such as ordinarily employed in the incandescent lamp industry, the filament being passed through the gettering cup in the usual manner and through a baking oven preferably in the form of a tube furnace in which the coating is baked onto the metallic core. An atmosphere of carbon dioxide is provided for the furnace to prevent decomposition of the carbonate coating and the temperature of the furnace is adjusted to about 1000 C.

Due to the tendency of the carbonates to settle out of the suspension, it is necessary to agitate the solution continuously, such agitation being accomplished with the apparatus set forth above by the rotation of the gettering wheel.

During the/baking of the coating the potassium 'perinan anate serves in some manner not fully understood to bind the coating to the wire and prevent the same from flaking or-rubbing off during the subsequent handling. The binding action may be due to the tendency of the soluble salt, that is. the potassium permanganate, to crystallize out of the suspension in which it is in solution and due to the formation of such crystais'to assist in bonding the coating to the Wire. The crystal so formed would also strengthen and toughen the coating. Potassium permanganate has a needle-like employing potassium permanganate crystal structure of such form that the ad- .jac'ent crystals tend to' interlock with each other and thereb form a mass which is strong and capab e of holdingithe carbonates firmly to the metal core and While the potassium perman anate is decomposed by the baking heat it is thought that the beneficial influence of the crystal form may be retained in thecompletedcoating.

Another possible explanation of the binding action is the fusion of the soluble salt or. of its products of decomposition when heat is applied thereto, resultlng in the roduotion of afused mass in which the car onates are held by a mechanical bond in such manner that they resist abrasion. \Vhatever the true explanation, the fact remains that when or equivalentsalts a much better bonding of the coating material onto the wire results.

While potassium permanganate has been specified as the preferred material it is to be understood that other soluble; salts which crystallize out of solution or which have a fusion temperature below the baking temperature of the coating and which exert no deleterious influence on the emission of the cathode may be employed and we do not desire to be limited to the specific material suggested, nor to the exact proportions specified since many changes may be made there in without departing from the invention.

What is claimed is:

1'.'A coating composition-for the production of thermionically active bodies comprising a compound of alkaline earth metal or metals in a suspending medium containing potassium permanganate.

2. A coating composition fOllZllB production of thermionically active bodies comprising a suspension of alkaline earth carbonates having in solution potassium permanganate.

3. A coating composition for the production of thermionically active bodies comprising amixture of the carbonates of barium and strontium in an aqueous suspension con taining a small percentage of potassium permanganate.

4. The method of producing thermionically active bodies which comprises coating ametallic base member with a suspension of a compound of an alkaline earth metal containing a soluble salt and baking the coating onto the filament ata temperature sufficient to crystallize out said salt and fuse the same. y Y

5. The method of producing thermionically active bodies which comprises coating a metallic base member with a suspension of a compound of an alkaline earth metal containing potassium. permanganate.

In testimony whereof, We have hereunto subscribed our names this 14th day of July,

WILLIAM BENJAMIN GERO. GEORGE WILSON HALLOOK. 

